Active metal anodes are those anodic metals such as lithium, calcium, magnesium, aluminum, and the like which, in aqueous electrolytes corrode, explosively releasing hydrogen and forming corresponding metal hydroxides. The term active metals as used herein encompasses the group of metals which displace hydrogen from aqueous solutions. They are a useful class of active metal anodes and particularly useful of this class are the light metal anodes.
Light metal anodes with halogen cathodes form an interesting group of active materials for fuel cells and semifuel cells and are characterized by high voltages as will appear from the following table:
TABLE I ______________________________________ CALCULATED CELL VOLTAGES FOR DIFFERENT ACTIVE METAL ANODES AND HALOGEN CATHODE COUPLES ______________________________________ CALCULATED CELL VOLTAGES CELL (VOLTS) ______________________________________ Lithium-chlorine 3.98 Lithium-bromine 3.52 Lithium-iodine 2.77 Calcium-chlorine 3.84 Calcium-bromine 3.40 Calcium-iodine 1.98 Magnesium-chlorine 3.04 Magnesium-bromine 2.58 Magnesium-iodine 1.85 Aluminum-chlorine 2.20 Aluminum-bromine 1.74 ______________________________________
Based on the chemistry of the elements utilized in the cells listed in Table I, it is obvious, that when it is attempted to use aqueous electrolytes for these cells, there are serious limitations associated with the decomposition of water. For example, active anode materials such as lithium, calcium, magnesium and aluminum and the like, corrode in aqueous electrolyte systems, explosively evolving hydrogen in forming the corresponding metal hydroxides. On the other hand, the halogens, the cathode active fuels, react with aqueous electrolytes to form halogen compounds. For example, when the halogen is chlorine, the compounds formed are chlorides, hypochlorites, and chlorates, depending upon the pH and the temperature conditions. For these reasons, it has not been possible to develop aqueous electrolyte fuel cells embodying these elements.
To overcome the difficulties encountered with aqueous electrolytes, attempts have been made to develop cells utilizing the couples shown in Table I in molten salt media. Some success has been achieved in this direction. It is obvious however, that the operation of cells employing molten salt electrolytes requires high temperatures and thus the cells cannot readily function under room temperature conditions.
It has also been proposed to provide active metal halogen fuel cell systems utilizing nonaqueous, organic electrolytes so that the cells can be operated under room temperature conditions and in the absence of detrimental gas evolution. These nonaqueous organic electrolytes are based upon certain organic solvents in which ionizable salts have been dissolved to obtain electrical conductivity and electrode compatibility.
Among active metal anodes suitable for the purpose of the present invention are lithium, calcium, magnesium, aluminum, and the like. These may be used with or without amalgamation with mercury. The cathodes in such cells are halogens selected from the group consisting of chlorine, bromine, and iodine operating at porous, inert electrodes such as porous carbon or graphite or halogen permeable teflonized membranes.
The novel electrolytes employed for cells are based upon organic solvents such as dimethyl formamide, dimethyl sulfoxide, methyl and butyl formates, ethylene and propylene carbonate, gammabutyrolactone, and similar polar solvents. Also useful are ethers and esters such as tetrahydrofuran, 1,2 -dimethoxy ethane, methyl and ethyl carbonates and acetates, and the like. In these solvents are dissolved inorganic ionizanble salts which are compatible with the halogen cathodes and the active metal anodes. Preferred are the halides such as the chlorides, bromides, and iodides; perchlorates; tetrachloroaluminates; tetrafluoroborates; hexafluorophosphates; and hexafluoroarsenates, of lithium, sodium, potassium, magnesium, or aluminum. In a presently available fuel cell of this type, halogens and active metals are utilized in a device wherein the halogens are reduced on a graphite cloth in an organic electrolyte to active metal halide salts. The halogen, therein described is admitted to the system from the outside and transferred to the graphite cloth cathode element with appropriate hardware.
Several problems encountered with these fuel cells were circumvented by mechanical devices. The halogen dissolved in the organic solvent is led to the graphite cathode element for introduction into cells only at a rate corresponding to the rate of which it is being consumed by the electrochemical reduction within the cells. If this rate is exceeded there is a danger that the elemental halogen will enter into electrolyte and directly react with the anode metal. In addition, it was found that the active metal halide salt, produced at the graphite cathode element by reduction of the halogen, displayed only limited solubility in this solvent and tended to separate out on the graphite. This reduced the effective surface area of the cathode element caused rapid polarization during cell operation.
The present invention is directed to an improvement by which these among other difficulties may be overcome.
It is, therefore, an object of this invention to provide a novel electrolyte which permits the graphite fuel cell cathode elements, in which the halogen is reduced, to function without becoming fouled or coated with the salt formed as a product of the cell reaction.
It is a further object to provide a halogen-active metal fuel cell and an organic electrolyte which will permit the anode to function efficiently even though there may be unreacted halogen dissolved in the electrolyte.